21 



averaging about 200,000,000, and reaching 1,697,000,000 August 9, 

 1898. The temperatures during these pulses are above 60, and 

 the period of the maximum comes toward the close of that of max- 

 imum summer temperatures, and sometimes in the autumn decline 

 (Pt. I., PL XI. and XII.), when low and often stable jriver-levels 

 usually prevail. A vernal and an early autumnal pulse are thus 

 both present in the distribution of this species. It is not improb- 

 able that other species than the one named have been included in 

 the enumeration along with it on account of the small size and lack 

 of striking characteristics. There are suggestions of recurrent 

 pulses at intervals of 2-6 weeks in the records (Table I.). 



Oscillatoria spp. Average number, 15,431 (filter-paper, 637,692). 

 The probable inclusion of several species in the sums under this 

 heading may account in part for the irregularity of the seasonal 

 curve. Oscillatoria has appeared in every month of the year, 

 though the occurrences were most frequent in the period from July 

 till the first of October. The numbers are exceedingly irregular 

 and variable, and the pulses of numbers seem to attend the initial 

 stage of floods following stable conditions. Thus, while these 

 organisms occurred but singly or sparingly in the plankton during 

 the autumn of 1897, they rose to 277,200 with the flood of January 

 11, 1898, doubtless torn loose by the current from the bottom- 

 their normal habitat. They are thus usually adventitious addi- 

 tions to the plankton. Their frequent irruption into the plankton 

 during midsummer and early autumn, and to some extent at other 

 times, is due in part to the evolution of marsh gas in the detritus on 

 the bottom. This breaks up the mats of Oscillatoria which coat 

 the bottom and distributes them through the upper levels, where 

 they remain in suspension for some time. This phenomenon is 

 more prevalent in the marshy backwaters than it is in the river. 

 Flood invasion in midsummer into the backwaters, such as Quiver 

 Lake, is wont to cause there stagnation and great increase in Oscil- 

 latoria, which to some extent enters the river with the run-off of the 

 flood. Movements in the water and the evolution of marsh gas are 

 thus principally responsible for the presence of Oscillatoria in the 

 plankton. It still remains possible that its flotation during periods 

 of optimum conditions of growth may be due to internal physio- 

 logical conditions which lower the specific gravity of the organism. 

 Its great abundance at times in upper levels in the backwaters sug- 



